1. Field of the Invention
This invention relates to a well tool assembly in subterranean oil and gas wells and, more specifically, to a seal assembly for use in the bore of a well tool such as a packer.
2. Description of the Prior Art
Downhole oil tools, such as packers, employed in oil and gas wells are commonly used with seal assemblies in tubing seal receptacles which can be attached to the tubing string which extends to the top of the well. In these conventional tubing seal receptacles, a seal stack, normally comprising an axial array of individual annular seals having a generally V-shaped cross-sectional configuration, is employed to establish sealing integrity between the tubular seal receptacle and the honed surface of the bore of a downhole tool. It is common practice to insert or stab-in a seal assembly on a tubing seal receptacle into the bore of a downhole tool such as a packer which has previously been anchored in position in the well bore. Under conventional environmental conditions within a well bore, these V-shaped seals can be fabricated from conventional elastomeric sealing materials such as nitrile rubber. Although problems can be encountered with inserting or stabbing such conventional seals into a packer bore, an exceptable degree of success has been achieved in establishing sealing integrity with the bore of a packer using these conventional seal assemblies.
Despite the reliability of conventional seal assemblies under normal well conditions, these conventional seals do not establish satisfactory sealing integrity under hostile conditions, such as high temperatures and in the presence of a corrosive chemical environment or other hostile conditions which may be encountered at a subsurface location in a well bore. High-performance sealing elements composed of materials which can withstand these hostile environmental conditions have been utilized. For example, certain elastomeric materials such as ethylene-propylene terpolymer elements, propylene tetrafluoroethylene, perfluoroelastomer, fluorelastomer elements and polytetrofluoroethylene elements perform satisfactorily under hostile conditions.
Despite the excellent sealing properties exhibited by these elastomeric materials under hostile environmental conditions, much difficulty has been encountered in actually inserting seals formed using these elements into the cylindrical bore of a downhole well tool. It has been found that seals formed from these compositions can encounter severe stab-in damage, sometimes resulting in the destruction of the annular seal elements, when seals mounted on a tubing seal receptacle are inserted into the bore of a well packer. It has been speculated that one source of these problems is the expansion or swell of these high-performance sealing elements as the tubing seal receptacle is lowered from the surface to the location of the well tool anchored in the well. For example, excessive thermal expansion can occur in certain elements. Other elements can exhibit significant swell or expansion when exposed to fluids which may be present within a well. For example, in certain wells using petroleum-based drilling fluids, the volume of the high performance sealing elements increases substantially.